This invention relates to structural connections between braces, beams and columns, using parallel gusset plate technology. As taught herein, gusset plates may be used to connect a brace, column and beam or, also, to connect a brace to a beam or a column. The use of gusset plates to connect beams to columns was taught in patent U.S. Pat. No. 5,660,017, mentioned above. This invention improves upon the structural connections taught in that patent, by reconfiguring the parallel gusset plates to receive diagonal braces. Thus, wherein the brace, column and beam are connected by parallel gusset plate, the system is a xe2x80x9cdualxe2x80x9d system because it uses gusset plates to attach both beams and braces to columns, thereby combining, interactively, a structurally braced lateral load resisting connection system with a structural moment resisting frame connection system. Similarly, wherein the brace and column only are connected by parallel gusset plates, the system is a special braced system because it acts alone to resist lateral loads.
The most commonly used braces, in this invention, are those known as wide-flanged xe2x80x9cHxe2x80x9d braces. Such braces have two wide flanges connected to each other by a web. The beams and columns most commonly used are xe2x80x9cHxe2x80x9d beams and columns, having two flanges and a web interconnecting them. However, other shapes may be used for brace, beam or column, or any combination thereof. Tube shapes and built-up box shapes are commonly known and used. It is to be appreciated that a box shape may be considered to have two flanges and two webs, acting in any principal direction, with the flanges in one principal direction acting as webs in the other principal direction, when loaded biaxially. Similarly, a built-up cruciform column may be used. Such cruciform column has four flanges and two webs which cross each other, described and discussed hereinafter, which flanges combine to provide significant stiffness and strength in each principal direction.
This invention is most useful in construction of single and multiple story buildings having a framework of structural steel. It is useful in either new construction or in retrofit construction of steel frame buildings to create a structure with both increased ductility and lateral stiffness.
It is to be appreciated that such joint connections would be useful in bridges and other structures using steel beams.
It has been found that substantial improvement is needed in the moment-resisting capabilities of beam-to-column connections in prior, structural steel buildings. Continuing and similar experience has been recently gained from both laboratory testing and from earthquakes, high winds, hurricanes, tornadoes, blasts, explosions and various other severe loading conditions which have happened, and which will continue to happen when using prior brace-to-column and prior brace-to-beam structural connections. Such loading conditions place similar demands on braced structural connection systems as they do on moment-resisting frame structural connection systems, which severe loading conditions in the past, have resulted in brittle fracture of both connection weld metal and base metal.
It is now common knowledge that prior beam-to-column connections and brace-to-column and brace-to-beam connections, which often used complete penetration welded joints between beam flange and column flange, and between the brace flange and the column flange and between the brace flange and the beam flange are not adequate and are susceptible to brittle fracture of the connection elements and the base metal, under severe loading conditions. The old, traditional, connection technology simply does not provide the needed strength and ductility required to withstand extreme loading conditions.
The prior art teaches numerous ways to connect beams and braces to columns. Previously, the common brace-to-beam and brace-to-column connection has been through the use of a single gusset plate welded or bolted to brace, beam and column. Other common brace-to-beam and brace-to-column connections used previously, involve welding at a skewed angle, the brace flanges directly to the faces of the column and beam flanges, respectively, using large, highly-restrained, full-penetration, single-bevel groove welds. This connection may actually be more vulnerable to brittle fracture than its common previously used moment-resisting frame beam-to-column connection counterpart, in part due to a more restricted access for welding.
In the prior beam-to-column connection, the beam has often had the ends of its top and bottom flanges welded to one flange, or face, of the column by large, highly-restrained, full-penetration, single bevel groove welds.
There has been partial or complete failure of the highly-restrained welds between the beam flange and the column flange, either by a crack in the weld itself or a crack along the heat affected zone of the column flange, and/or a crack in the column flange base metal, pulling a divot of column steel from the face of the column flange.
In addition, failures between the beam flange and column flange have resulted in shear failure of the high strength bolts connecting the shear tabs to the web of the beam for the support of the gravity loads. Vertical loads, that is, the weight of the floors and gravity loads acting on the floors, are commonly carried by vertical shear tabs. Each such shear tab is vertically disposed and is welded to the face of the column and bolted or welded to the web of the beam, at the end of the beam which is nearest the column, using high-strength bolts.
Subsequent attempts by the building industry to improve beam-to-column connections and brace-to-column and brace-to-beam connections still rely on post-yield straining of large, highly-restrained, full-penetration, single-bevel grooves welded under field conditions. Such highly-restrained welds do not provide a reliable mechanism for dissipation of earthquake energy, explosion or blast energy, or other large forces, and can lead to brittle fracture of the weld and the column. Such brittle fracture shows that the design violates the ductile design intent of the Uniform Building Code, for both moment-resisting frame connection systems and special braced frame connection systems.
Of course, there are other requirements to be met, some of which are set forth in AISC (American Institute of Steel Construction) publications, including, but not limited to, the LRFD (Load and Resistance Factor Design) specifications and ASTM (American Society for Testing and Materials) publications, as well as ASME (American Society of Mechanical Engineers).
Skilled in the art structural engineers, designing strengthened structural steel buildings are familiar with the various design requirements set forth in those publications and the various State and local building codes which may be involved.
Contrary to prior beam-to-column structural joint connections, and brace-to-column and brace-to-beam structural joint connections, the present invention, by taking advantage of parallel gusset plate technology, does not rely heavily on post-yield straining of the joint connection.
In the case of earthquakes and explosions, greater strength and ductility are particularly desirable in resisting sizeable loads in both the lateral and the vertical directions.
Parallel gusset plate technology, or, simply, gusset plate technology, as taught in the patent mentioned above, has been a substantial step forward in strengthening the beam-to-column connections in a building comprised of structural steel beams and columns. Two parallel gusset plates are attached on opposite sides of a column and attached on opposite sides of a beam, to connect the beam to the column.
Engineering analysis, design and testing have determined that the advancement provided by the parallel gusset plate technology can be further advanced and improved by using gusset plate technology to add braces in the form of a dual structural system, or to connect a brace to either a column or a beam without a moment connected beam-to-column connection at the same location, to better resist and withstand the sizeable loads which are placed on a building structure in a disaster.
Braces have long been used to add both vertical and lateral stiffness and to strengthen steel construction, in buildings and truss style bridges. But prior art has not used parallel gusset plate technology, for connecting and combining both braces and beams to columns. Nor has the prior art used welds to connect a brace, (which extends between parallel gusset plates welded to a column), to the parallel gusset plates either directly or through the use of cover plates. Structural engineers are aware that mere addition of braces could cause too much stiffness in the structure, when, as a matter of fact, substantial ductility and yielding is required in order to absorb the tremendous energy involved in heavy, disastrous overloading from, for example, an earthquake, a hurricane or an explosion. Without the ductility and yielding, the brace would be susceptible to buckling and failing in compression.
Applicant has previously developed parallel gusset plate technology to strengthen beam-to-column connections and has obtained the above-mentioned patent U.S. Pat. No. 5,660,017, entitled Steel Moment Resisting Frame Beam-To-Column Connections and, further, has filed patent application Ser. No. 09/141,714, entitled Moment Resisting, Beam-To-Column Connection. Those patent documents disclose a great deal about strengthening beam-to-column connections through the use of parallel gusset plates. The teachings therein are incorporated herein by reference.
As pointed out previously herein and in the mentioned patent documents, such gusset plate technology also works well with tube shapes and with box shapes. For example, the box shapes may be a box brace, a box beam or a box column.
Columns are commonly strengthened by horizontal shear plates welded between flanges, on each side of the web of the column. Beams are strengthened by vertical shear plates, which also carry vertical loads in shear, which shear plates are fillet welded to beam flanges and the beam web as well as to the gusset plates. Column composite dynamic shear capability may also be increased, for xe2x80x9cHxe2x80x9d columns, by providing concrete in fill in the space between column flanges, on each side of the column""s web, wrapped using fiberglass jackets or, even using external jackets of structural steel, to provide ductile containment of the infill. For tube columns or box columns, containment of concrete infill is self-provided by the column""s own shape.
In general, in gusset plate technology, two parallel gusset plates connect a beam to a column, using fillet welds to fixedly attach the gusset plates with respect to the beam and using fillet welds to fixedly attach the gusset plates to a column. Where necessary, top and bottom cover plates are used to bridge the difference between the beam flange width and the width between the gusset plates, which is the width of the column flanges. Fillet welds are used between the cover plates and the flanges of the beam, and between the gusset plates and the column flanges. Alternatively, in the case of a beam whose flange width is as wide as the column flange, the flanges of the beam are welded directly to the gusset plates. Such gusset plate technology is eminently successful.
As mentioned hereinabove, disastrous events, such as earthquakes, high winds, hurricanes, tornadoes, blasts, explosions and the like cause severe loading on steel structural constructions. When braces are used in the structure, some of the severe loads are severe moment loads on the braces. This invention teaches a person skilled in the art, a structural joint comprised of column and beam connected by a pair of gusset plates. In addition, this invention teaches a column, beam and brace connected by the same gusset plates. Each of the gusset plates is fixedly attached with respect to the brace by a severe moment-resisting connection, that is, a connection strong enough to resist severe moment loads beyond normal gravity loading, such as those loads caused by the above-mentioned disastrous events. Thus, in the joint connection, the same gusset plates which provide fixed attachment of the beam with respect to the column are used to provide the fixed attachment of the brace with respect to the joint connection of the beam and the column.
The invention herein enhances the prior-used and prior-disclosed parallel gusset plate, beam-to-column technology by fixedly attaching braces directly to the parallel gusset plates or to intermediate bridge plates which, in turn, are connected to the gusset plates.
The braces may be attached by bolts, rivets or by welding. If welded, the preferred weld is the fillet weld, which, in this invention, is usually directed in the same direction as the line of greatest strain. The brace, in the preferred embodiment is bolted or, possibly, riveted, to the gusset plates, using oversize holes, thus providing a yielding, energy-dissipating connection between the brace and the beam and between the brace and the column.
If the brace is fillet welded to the parallel gusset plates, the fillet weld carries the load along its longitudinal length in shear, rather than across its narrow width in tension, which transfer in shear is inherently robust and ductile in performance.
The addition of braces to parallel gusset plate technology, as taught in the preferred form of this invention, creates a structural dual system when combined with a beam-to-column connection at the same location. Thus, the structural system of the invention provides even greater strength, frame continuity, and structural redundancy and stability in both horizontal and vertical directions, without compromising connection system ductility.
The structural redundancy and frame continuity of this invention, using parallel gusset plate technology in a dual system application are provided by the creation of monolithic steel frames which combine and integrate complementing structural lateral load resisting systems using parallel gusset plates that are fixedly attached with respect to the concentric intersection of braces, beams and columns, using fillet welded construction to join the connection elements.
The joint connections taught herein comply with the seismic provisions for steel frame buildings, issued by the International Conference of Building Officials and the American Institute of Steel Construction.
One significant feature of the gusset plate technology, which is enhanced further by the brace-to-column parallel gusset plate technology is that it is cost-effective. By providing stiffer beam-to-column connections, combined with a braced structural system which is also made stiffer by the use of gusset plate technology, lighter steel beams, columns and braces can be used, while still providing greater overall ductility and lateral stiffness in the structure of the building, compared to prior structures.
The same stiffness advantage is applicable when only braces are connected to columns using brace-to-column parallel gusset plate technology.
Just as with the beam-to-column gusset plate technology, the brace-to-column and brace-to-beam gusset plate connection systems taught herein may be fabricated in the shop under controlled conditions for placement in a new structure, or placed in the field during retrofitting of a previously-built structure. Shop fabrication provides for better quality construction by reason of better control of the manufacturing process and easier access to and handling of all parts of the connection. The invention makes use of fillet welds in some embodiments of the brace-to-gusset plate connection. The invention still takes advantage of the beam-to-column parallel gusset plate connection which uses fillet welds and which are better and more easily made under shop conditions. However, the invention can suitably be manufactured in the field.
Splice connections are commonly used in the field to insert stub sections. In the shop, where quality is more easily and better controlled, gusset plates can be attached to a column, a beam stub, and/or a brace stub, each of which stubs are suitably spliced into the link beam and/or link brace structure in the field. Such splice connections are, preferably, located at structural points of reduced flexural stress. That is, the splice connections are commonly located at some distance from the brace-to-column and beam-to-column connection.
Welds are commonly used to structurally connect brace stubs to link braces and to connect beam stubs to the link beams. Such welds are usually complete joint penetration groove welds using a flange-to-flange and web-to-web welded butt joint configuration. If done properly, the welded joint is as strong or is stronger than the non-welded portion of the beam or column. In other embodiments, bolted or riveted plates may be used to connect link beams or braces.
The steel elements used in most buildings having steel frameworks, are likely to be what is known as A-36 specification, structural steel, or A572 Grade 50 specification, structural steel, except for the bolts and washers.
In bolting the braces to the gusset plates, oversize bolt holes facilitate construction and provide energy dissipating mechanisms through bolt slippage at high stress levels. Such slippage has been determined capable of dissipating substantial additional energy in the event of seismic overload, tornadoes or other severe stress being placed on the structure. Bolts which are used in most steel construction and in this invention are most commonly slip-critical A490 or A325 tension-controlled bolts.
In bolting, it is common practice to drill the bolt holes to be slightly oversized. The bolts, for example, that would be used in this invention are, of course, high-strength bolts. The oversize bolt holes allow easier fitting together of the structural elements and, further, provide an energy dissipation mechanism through bolt slippage at high stress levels. Washers are commonly included, in accordance with customary practice, although washers are not shown in the various Figures herein, because they are so relatively small in size. It would be expected that all bolting would include washers. The bolts used throughout the invention are high strength bolts which can be field or shop bolted. The bolts in the oversized holes are tightened to be slip-critical, meaning the adjoined metal plates cannot slip or move under designed load.
The prior beam-to-column gusset plate invention, as taught in patent U.S. Pat. No. 5,660,017, referred to above, utilizes unrestrained, inherently-ductile fabrication by fillet welds which are loaded principally in shear in the directions of great strain, by parallel gusset plates which connect the beams to the columns and by the elimination of prior, highly-restrained, groove welds between beam flange and column flange.
This invention continues with that concept by adding, or, substituting (as applicable), braces to such parallel gusset plate construction, with welds which run in the direction of great strain, that is, along the longitudinal axis of the brace. In the preferred embodiment, the braces lie diagonally in the vertical planes of the building. In other embodiments such braces may be connected and disposed to lie diagonally in horizontal planes within the building. Of course, it is to be recognized that in some embodiments, in tower construction or triangular construction, the planes may not be truly vertical.
Vertical stability, stability in the vertical plane, is achieved by the great strength of the gusset plates and their strong connection to column, beam and brace. The joint connections of the present invention, which adds diagonal braces to the gusset plates, can be designed to withstand a load that is greater than the plastic moment capacity of the connected beam.
Lateral stability, stability in the horizontal plane, is achieved in the instant invention, by the structural frame of the building in such horizontal plane. That is, the beams connecting each column to its adjoining columns and beams provide the primary resistance to moments in the horizontal plane.
It has been pointed out above, that rivets or welds might be used in place of the bolts, in attaching braces to gusset plates. However, bolts are preferred for use in such structural joints. The term xe2x80x9cfastenerxe2x80x9d or xe2x80x9cfastenersxe2x80x9d herein is intended to include either or both bolts and rivets. Such xe2x80x9cfastenersxe2x80x9d allow slippage and provide an energy dissipating mechanism. xe2x80x9cFastenedxe2x80x9d is intended to include attachment by use of xe2x80x9cfastenersxe2x80x9d. xe2x80x9cAttachedxe2x80x9d herein, includes xe2x80x9cfastenedxe2x80x9d, (bolted or riveted), and xe2x80x9cweldedxe2x80x9d. xe2x80x9cWeldsxe2x80x9d and xe2x80x9cweldedxe2x80x9d includes fillet welds and, also, complete joint penetration, groove welds and other welds, provided they are suitable to meet the design requirements of the building.
One end of a brace may be connected to one beam-to-column set of gusset plates, and the other end of the brace may be connected diagonally to another beam-to-column set of gusset plates.
However, it is to be appreciated that while one end of a diagonal brace may be connected to one beam-to-column set of gusset plates, the other end of the diagonal brace may be connected or fixedly attached otherwise, that is, to the beam a selected distance from the beam-to-column connection. This selected distance provides an xe2x80x9ceccentric linkxe2x80x9d, (a section of the beam between its connection to the brace and its connection to the column), which xe2x80x9ceccentric linkxe2x80x9d serves to provide energy absorption through ductile shear and bending deformation, to allow columns, joints and connections to remain stable. That is, the linking beam, to which the brace is attached, bends and provides ductility and reserve for greater energy absorption capability.
Parallel gusset plate technology also permits usage of xe2x80x9cconcentricxe2x80x9d braced frames, (wherein two braces meet at a beam and use a single set of two parallel gusset plates to attach braces and beam), or in xe2x80x9ceccentricxe2x80x9d braced frames (wherein two braces are attached to a beam by two, separate sets of two parallel gusset plates, a fixed distance apartxe2x80x94a predesigned xe2x80x9ceccentric linkxe2x80x9d distance).
It is, therefore, an object of this invention to provide substantially strengthened joint connections using parallel gusset plate technology to connect brace to beam and column, brace to column and brace to beam.
Another object of this invention is to provide a brace-to-column, brace-to-beam and beam-to-column joint connection using a single set of parallel gusset plates.
A further object of this invention is to provide a brace-to-column and brace-to-beam joint connection which provides an energy dissipating mechanism through bolt slippage at high stress levels, using parallel gusset plates.
Still another object of this invention is to provide structural redundancy by creating a dual, lateral load resisting connection system that connects brace to column and beam to column, using mutually common parallel gusset plates, without reliance on beam flange-to-column flange welded connections.
Still another object of the invention is to provide a structural brace-to-column and beam-to-column structural joint connection which eliminates post-yield straining of large highly-restrained, full-penetration groove welds.
A still further object of this invention is to provide suitable structural brace connections in structural buildings using either xe2x80x9ceccentric linksxe2x80x9d or using concentrically braced framing configurations.
Further objects and features will become apparent from the following drawings and description.